1. Field of the Invention
This invention relates to forming sealed seams in moisture barriers for cables and, more particularly, to methods of and apparatus for causing overlapping portions of a longitudinally formed moisture barrier wrapped about an advancing cable core to be bonded continuously adhesively to foreclose the ingress of moisture into the cable core.
2. Description of the Prior Art
It is well known that the diffusion of water vapor in a uni-directional manner through a cable jacket into the core will have a detrimental effect on the transmission characteristics of the cable. It is also well known that a metallic tube surrounding the cable core provides an effective barrier against moisture diffusion into the cable core. Since most communications cables require an electrically conductive, metallic sheath, to shield against external electrical signals, it is usually desirable to incorporate the shielding function with the moisture barrier function in one single layer of metal.
The tube which comprises the moisture barrier is generally made from a single strip or tape of metal wrapped longtiudinally about the cable core to form an overlap seam. Moreover, in order to impart greater flexibility to the cable, the moisture barrier is frequently corrugated prior to being wrapped about the core.
The effectiveness of the tube as a moisture barrier is enhanced greatly if the resulting overlapping seam is sealed. A most effective seam seal is one in which a metal-to-metal bond exists, such as a welded or soldered seam. See, for example, U.S. Pat. Nos. 3,172,388 and 3,203,085, incorporated by reference hereinto. Norwegian Pat. No. 96,555 shows a device which extends between a portion of a shield and a core to support the shield during the application of closing forces thereto.
Problems arise when attempting to weld or solder a seam because of the high temperatures involved in most welding processes. The materials which make up most communications cables are temperature-sensitive and easily damaged if over-heated.
Problems arise in attempting to seal the seam because the material from which it is generally preferred to construct the barrier is aluminum. Because of its highly reactive nature, the aluminum maintains almost consistently an oxide film which virtually precludes continuous soldering as a joining technique for the overlapped seam.
Further, the strip or tape from which the moisture barrier is formed is extremely thin, e.g., on the order of 4 to 6 mils. The thinness of the barrier coupled with the typical corrugated configuration render undesirable the use of welding or soldering for sealing the seam.
It has been found that a sealed moisture barrier can be obtained by adhesively joining the overlapped seam. This may be accomplished by using adhesive-copolymer aluminum laminates at least for those portions of the barrier which form the seam. The application of adhesive copolymers in laminates is discussed by B. Wargotz in an article "Environmental Stability of Ethylene-Acrylic Acid Adhesive Copolymers Bonded to Metal Substrates" pulished in Vol. 12 of the Journal of Applied Polymer Science, pages 1873-1888 (1968) and incorporated by reference hereinto. See also U.S. Pat. No. 3,681,515, and "Notes on the Manufacture of Bonded Jacket Cables and Plastic Pipes Lined wtih Zetabon" published by the Dow Chemical Company of Midland, Michigan in 1970.
The art abounds with patents relating to moisture barriers or tube-making with bonded-type longitudinally formed, overlapped seams. In U.S. Pats. No. 3,574,016 issued to R. R. Wahlberg, an overlapped seam is sealed with an acrylic acid ethylene copolymer tape which has been inserted into the seam and subsequently heated by the extrusion of a thermoplastic jacket over the metallic strip. In an alternate embodiment in that patent, there is shown a cable sheath in which the inside surface of the metallic strip is precoated with an adhesive copolymer. A still further embodiment discloses the combining of a precoating of the shield with a film of adhesive copolymer on the inwardly facing major surface with the introduction of a strip of adhesive material into the seam portion.
Another patent, U.S. Pat. No. 3,703,605, issued to M. R. Dembiak and G. W. Webster, shows a moisture barrier coated across the entire transverse width on the outwardly facing surface and along an edge portion or the portion of the strip which forms the overlapped seam. The compositions of the coatings are different to permit a thin striping of the inwardly facing surface in order to avoid specific manufacturing problems.
F. F. Polizzano U.S. Pat. Nos. 3,504,102 and 3,575,748 and O. G. Garner, Pat. No. 3,332,133 show coated moisture barriers with overlap seams. U.S. Pat. No. 3,409,734 issued to H. Devine et al. discloses a narrow band of adhesive applied to the inside surface of an outer layer of a tape enclosing a metal strand and bonded to the underlying tape surface so as to leave a fine strip of the tape adjacent to the edge free from adhesive. See also U.S. Pat. No. 3,260,636 issued to H. Witzenmann, U.S. Pat. No. 3,471,350 issued to R. V. O'Berry et. al. and I. H. Marantz U.S. Pat. No. 3,558,378. Shigekazu Takeda U.S. Pat. No. 3,597,292 discloses a nozzle distributing a molten resin in the longitudinal overlapped seam portion of synthetic resin fabrics.
The related art of pipe-making includes, for example, F. W. Yeager, U.S. Pat. No. 2,044,456, U.S. Pat. No. 775,541 issued to J. A. McConnell and U.S. Pat. No. 2,897,875.
In the can making art, U.S. Pat. No. 3,066,063 shows a web brought into a tubular configuration with a seam sealing ribbon such as a fiber coated with a suitable adhesive, thin gage metal, or a synthetic resin of flexible coherent material extruded or fed as a preformed ribbon into engagement with portions of the overlapped seam portions and rollers. The rollers form the semi-molten ribbon into an S-shape about the longitudinal seam edges. Subsequently, rollers force the seam portions into intimate superposed relation while the web is being cooled.
Despite the numerous processes for forming a longitudinal seam having integrity against moisture diffusion, problems are yet experienced in obtaining consistently a reliably sealed seam in cable manufacture, especially when the metallic tape is corrugated. With the trend toward an increased number of corrugations per unit length, the misregistration of corrugations at the overlapped seam causes excessively deep valleys. This results in non-engagement of the adhesive on some of the opposing faces, thereby providing undesirably gaps in the seam.
The use of a preformed or semi-molten tape advanced into engagement with portions of an overlap seam has been found not to be effective in producing cables having a corrugated moisture barrier. Forces required to insure that the tape in either form is deformed sufficiently to fill any misregistration voids have the undesirable effect of tending to flatten the corrugations. Moreover, any forming of such a tape in a configuration, for example, to seal the longitudinal edges, may cause problems because of the alternating peaks and valleys of a corrugated barrier.